Click “Sign up to get your first AppID”.
For “How are you planning to use the Wolfram|Alpha API?”, enter “Nightbot command”.
For “May we contact you about upgrading to a paid API plan?”, select “No”.
Agree to the terms of use, and click “Sign up”.

$(eval s=decodeURIComponent("$(querystring)");try{r=JSON.parse(s),r=r&&r.queryresult,r?(e=r.error)?"input parameter not present in query"==e.msg?"No query":e.msg||"Error":r.success?(p=r.pods)&&p[0]?(b=p[0].subpods)&&b[0]?(b[0].plaintext||"Query result has no text").replace(/(?<=[^ ])(?=\()|(?<=\))(?=[^ ])/g," ").substr(0,400):"Query result has no subpod":"Query has no result":"Query unsuccessful":"No response"}catch(a){`Failed to parse Wolfram|Alpha result: ${a.message}: ${s}`.substr(0,400)})

Click “Submit”.

Now you will have a !convert command.
If you want, you can name this command something more general, because it can answer any question that Wolfram|Alpha can answer.